Unable to subscribe to Office 365 IMAP folders from MUA since 2022-11-01

I am not alone; several users of pre-existing accounts on our tenant have been affected around the migration date (local to cloud) and have reported this.

The real number of affected accounts is unknown though, as many use Outlook over MAPI, or the O365 web-app to check their emails.

Apparently there are no issues with newly created user accounts, but this was not extensively tested.

There is indeed no evidence that the issue pertains to OAuth2.

However, my email client tried to connect automatically via the old authentication procedure on the day after the migration, before I changed the settings manually to authenticate over OAuth2. Perhaps this has messed something up in the configuration files on the server? - Just thought I should mention that.

For documentation: Set up of the affected email account on fresh installations of email clients on two different machines did not fix the issue for me. This indicates that the configuration/communication issue exists on the server (or tenant) and not on the client side.

This is purely speculation but I'm wondering if Microsoft broke IMAP again in a similar way on this occasion, issued a quick code patch to fix it, but didn't restart the relevant services properly - leaving some accounts to hang in a broken state with the defective code. Affected accounts could simply be the ones that happened to try and connect with IMAP within the time frame it took Microsoft to realise they had a problem.

I suspect the real cause is different from the hypothesized one.

The reason is that issues with my affected account arose not at the beginning of November,

but only later that month when our Exchange tenant was migrated from a local server to the Microsoft O365 cloud. During this procedure classic password authentication was also substituted for OAuth2.

The issues started immediately after.

I must have missed that point in your particular case. It certainly puts a different slant on it. What does your system administrator think of the issue? Did it affect all of the pre-existing accounts that were migrated? Have they had any problems with any new mail accounts created since the migration took place?

I'm still not convinced of any link to the Oauth2 change though. My affected and unaffected accounts were running Oauth2 long before the change. The Microsoft test script requires you to log in as the user you're testing at the start of the script and it is clearly using Oauth2 to do that. The only difference between an affected and an unaffected account is the server response to the LSUB commands.

I'd take that with a pinch of salt, as I'm sure they didn't provide any justification/evidence for why they think it might be a client side problem + as we know, Microsoft's own test script provided to me exhibits the exact same problems as the various affected MUAs too.

[...]

I suspect this will mysteriously resolve itself at some point.

Gotta love that transparency and open communication; makes you really trust the service provider whom you are dependent on.

This is purely speculation but I'm wondering if Microsoft broke IMAP again in a similar way on this occasion, issued a quick code patch to fix it, but didn't restart the relevant services properly - leaving some accounts to hang in a broken state with the defective code. Affected accounts could simply be the ones that happened to try and connect with IMAP within the time frame it took Microsoft to realise they had a problem.

I suspect the real cause is different from the hypothesized one.

The reason is that issues with my affected account arose not at the beginning of November,

but only later that month when our Exchange tenant was migrated from a local server to the Microsoft O365 cloud. During this procedure classic password authentication was also substituted for OAuth2.

The issues started immediately after.

Wow - I am flabbergasted by the "support's" incompetency.

But hey, let's just archive and forget about these bug reports; problem solved.

Only thing left to do now, is to wait for some forum agent's run-of-the-mill reply to be selected as the accepted answer and this thread to be locked.

When Microsoft "Archive" a case, I think that is just their strange terminology for what everyone else in the IT world calls "suspend". They have archived my case a couple of times when they weren't expecting any update for a few days.